"The preservation of the rule of law in our country demands a rapid investigation of the facts and a rigorous punishment of those responsible for this barbarous act."

Mrs Acioli had been on a death list, found in possession of a jailed militia leader this year, the Globo news website said.

Militias made up of off-duty police and firefighters have expanded in Rio's lawless slums in recent years, ousting drug gangs in turf battles over the lucrative supply of services such as cable TV and cooking gas.

"Patricia received threats for at least five years. She was considered a hard-line judge, always giving the maximum penalty," said her cousin, Humberto Nascimento.

Rio has been trying to tackle its reputation for violent crime as it prepares to co-host the soccer World Cup in 2014 and host the Olympic Games in 2016.